Digital projectors are widely used for business and classroom presentations, as well as for home entertainment applications. A digital projector receives a video signal from a host computer or other video source, such as a DVD player, and displays a graphic using a display unit within the digital projector in accordance with the video signal. Display units may include pixel displays such as liquid crystal display (LCD) chips, liquid crystal on silicon (LCOS) chips, or digital micromirror device (DMD) chips. The graphic is then projected from the display unit onto a projection screen using a light source and lens system, so as to form a projected image on the projection screen.
In ideal conditions, the digital projector projects the graphic onto the projection screen along a projection axis that is perpendicular to the projection screen to form an image on the projection screen. In such ideal conditions, there is minimal keystone distortion and the proportions of the image are generally similar to the proportions of the graphic displayed by the display unit.
Keystone distortion occurs when the projection axis is not perpendicular to the projection screen due to the position of the digital projector relative to the projection screen. Keystone distortion causes an image to have a stretched, often trapezoidal appearance on the projection screen. Thus, the proportions of an image with keystone distortion are not similar to the proportions of the graphic displayed by the display unit.
It has generally been considered to provide keystone correction capabilities in a digital projector. Consistent with keystone correction capabilities, a graphic is projected from the display unit onto a projection screen to form an image having keystone distortion. A transformation is then applied to the graphic so that the graphic is displayed on the display unit with a pre-distortion that is the inverse of the keystone distortion. As a result, when the pre-distorted graphic is projected onto the projection screen to form an image, the resulting projected image is aligned with the projection screen.
The transformation is often entered manually by the user, who selects the transformation in a trial-and-error process.